For the third part in my series about natural selection I’d like to address the concept of fitness. Thanks to the phrase “survival of the fittest”, fitness is quite a prominent idea in the popular perception of evolution. It was originally coined by Herbert Spencer after he read On the Origin of Species; Darwin adopted the phrase in later editions and it’s been popular ever since. Unfortunately, this glib phrase has often eclipsed a more accurate depiction of evolution, leading to some common misunderstandings. This confusion arises because “fitness” has a different meaning in evolutionary biology than it does in general usage. (Have a look at the earlier posts in this series if you haven’t already read them; the first was about the different modes of natural selection and the second discussed selection mechanisms, focusing on sexual selection in particular.)